The present invention provides unsaturated polyester resin compositions that contain one or more unsaturated polyesters, ethylenically unsaturated monomers that copolymerize with the unsaturated polymer, thermoplastic low-profile additives and compatible compounds.
Unsaturated polyester resin compositions are finding increased use in the automotive industry as sheet molding compound (SMC) formulations from which component parts especially body panels can be molded. The unsaturated polyester resin compositions contain, in addition to the unsaturated polyesters and monomer components, so-called "low-profile" additive components which are thermoplastic polymers that act to prevent undesirable shrinkage as the composition is being molded into a thermoset article. Low-profile additives are added to unsaturated polyester resin compositions in order to obtain a composition which can be used in a sheet molding formulation and molded into thermoset articles. The surfaces of the molded articles truly reflect the surface characteristics of the mold.
Two types of low-profile systems are commonly used commercially, one-pack and two-pack. In one-pack systems, the unsaturated polyester, monomer and low-profile additive components are mutually compatible, i.e., no gross separation occurs when a mixture of the components is allowed to stand. In contrast, two-pack systems form distinct phases if the components are allowed to stand after being mixed. As such, the components need to be mixed immediately prior to use. In both systems, phenomena occur that allow these resins to microscopically compensate for shrinkage.
It is the ability of the low-profile resins to compensate for shrinkage that leads to the usefulness of these resins. This shrinkage compensation is largely a result of a micro-phase separation that occurs in these unsaturated polyester resin systems. The micro-phase separation occurs during the cure phase for both the one-pack and two-pack systems. Prior to cure the low-profile additive is at least partly soluble in the polyester/monomer solution. As the polyester/monomer mixture crosslinks, the low-profile thermoplastic additive and copolymer (of polyester and monomer) become increasingly less compatible and a two-phase (domain-matrix) type morphology results. This micro-phase separation leads to the formation of a porous structure as the opposing internal stresses of thermal expansion and polymerization shrinkage occur. In many unsaturated polyester resin compositions the porous structure is a result of microfracturing of the curing resins which gives rise to void formation. Other unsaturated polyester resins have been developed which have essentially zero shrinkage and which, in fact, expand upon curing.
In addition to unsaturated polyester resins, the sheet molding compound formulations typically contain other ingredients including, for example, chemical thickeners. In such formations, a chemical thickener such as an alkaline material (for example, magnesium oxide or magnesium hydroxide) is added to an uncured polyester along with fillers, glass fiber, and other standard materials. The alkaline material interacts with the residual acidity in the polyester and, usually, the low-profile additive to increase the viscosity of the composition. This process is referred to as maturation and usually takes several days. If two-pack resin systems are used, care has to be taken to avoid gross phase separation. After the maturation process is complete, the thickened formulations are handlable and can easily be placed into compression molds either by hand or by machine.
Although the use of low-profile additives does effect some degree of improvement in the anti-shrinkage characteristics of the unsaturated polyester compositions, it has now been found that significant improvements in surface smoothness and processing characteristics can be achieved by adding a component which remains compatible during the curing of the unsaturated polyester and monomer.